[0001] The present invention relates to a process for preparing double-wall blow-molded
articles having a high stretching ratio and to the products so obtained. Such a process
is known from e.g. US-A-4,423,000.
[0002] Further DE-A-14 79 695 discloses a process for blow-molding a double-wall article
from a cylindrical parison, wherein the parison is blown after the mold has been closed.
[0003] DE-A-14 79 704 describes an extruding apparatus which extrudes directly a cylindrical
parison into a mold cavity, whereupon a piston is moved into the mold cavity.
[0004] More in particular, the present invention relates to a process for preparing double-wall
articles having a high stretching ratio obtained by forming,by means of blowing, a
thermoplastic extruded parson, which is contained in a modified mold for blow molding.
[0005] The term "double-wall blow-molded articles having a high stretching ratio", whenever
used in the present specification and in the claims, means formed bodies or articles
of any shape, either circular or polygonal, to be utilized in sectors such as transports,
household electrical apparatus and appliances, building, autos, telecommunications,
office machines, etc., as doors, covers, casings, in particular for refrigerators
or frizers, panels, containers, etc., where the stretching ratio, intended as the
ratio between surface of the formed article and starting surface of the tubular extruded
body included in the mold, is higher than 3 and preferably ranges from 3.5 to 4.
[0006] The technology utilized for producing blown articles having a high stretching ratio,
which is known as Stretch Blow Molding, is limited so far to the production of articles
having a strictly cylindrical geometry (nearly totally bioriented bottles for drinks
additioned with carbon anhydride).
[0007] By this method, a polymer extruded in a tubular shape (parison) or an injection molded
preform are stretched by means of a pliers system or by means of a piston (in the
case of the Injection Blow Molding) in order to impart to the articles, by means of
a bi-orientation of the polymeric chains, improved mechanical properties and a lower
permeability to gases.
[0008] The Applicant has now found a process which represents an extension of said technology
to articles having a complexer geometry, in which the stretch is obtained during the
mold closing step, by moving said mold.
[0009] Thus, object of the present invention is a process for preparing double-wall blow-molded
articles having a high stretching ratio.
[0010] This object is achieved by the features in claim 1.
[0011] Any thermoplastic polymer can be used to prepare the parison; illustrative examples
comprise: polystyrene, impact-resistant polystyrene, polystyrene modified with polar
monomers such as a acrylonitrile, styrene mixes such as ABS, SAN, etc., polyvinyl
chloride, high, mean and low density polyethylene, polypropylene, impact-resistant
polypropylene, ethylene/propylene copolymers, acrylic and/or methacrylic resins, polymethacrylates,
polyester resins such as PET, PBT, and the like.
[0012] Preferred products are polystyrene, impact-resistant polystyrene, styrene mixes,
polypropylene and impact-resistant polypropylene.
[0013] The parison, which has a preferably round section, has a thickness, which is a function
of the shape and size of the article to be produced, but generally ranges from 0.5
to 20 mm; furthermore the parison, to facilitate its thermoforming, is maintained
at a temperature, which is above the softening temperature of the polymer which it
is made of, generally higher than 100° C, preferably ranging from 120° to 200°C.
[0014] The parison is introduced into a mold, which reproduces in negative the shape of
the article to be produced. More in particular, the mold is composed of two parts.
A part, the concave one, is stiff, while the other is equipped with a movable piston,
which can be shaped so as to conform to the concavity of the other half-mold. As an
alternative, the piston can consists of a support, on which different templates can
be adapted, as a function of the shape of the concavity of the stiff part and, therefore,
as the function of the article to be produced.
[0015] The blow molding technique is known to those skilled in the art and is described,
for example, in Encyclopedia of Polymer Science and Engineering (Second Edition) Wiley
Interscience, 1985, Vol. 2. page 447. More in particular, the molding of the parison
according to the present invention comprises blowing air or an inert gas, such as
nitrogen, into said parson and simultaneously, or almost simultaneously introducing
the piston into the cavity of the stiff half-mold.
[0016] "Almost simultaneously" means that the piston can move also after a minimum time,
approximately ranging from 0.5 to 2 seconds, after the blowing begin.
[0017] In order to obtain a more homogeneous distribution or the thermoplastic material
inside the cavity, vacuum can be generated, during the molding step, in the concavity
of the first half-mold in order to enhance the stretching effect and a correct forming
of the outer walls of the article. At the end of the molding step, the article is
brought to a temperature suited to the drawing. The double-wall blown articles having
a high stretching ratio prepared by means of the above-illustrated process are, moreover,
a further object of the present invention.
[0018] According to the process of the present invention it is possible to produce double-wall
articles having a marked development of the surface perpendicularly to the parison
extrusion direction.
[0019] Furthermore, the use of a punching half-mold has the advantage of preventing a too
early adhesion of the walls of the extruded parison under forming, so permitting the
production of blown articles, in which the distance between outer walls and inner
walls is very little.
[0020] A further advantage deriving from the use of the present process is a considerable
reduction of the mold, which, permitting a greater opening of the mold-holding plates,
facilitates the drawing of the molded pieces and makes possible shorter closure times
and, by consequence, the use of materials having a lower melt strength.
[0021] A practical embodiment, which is given as an example but not as a limitation of the
present invention is illustrated in the attached drawing, in which:
- figure 1 is a sectional view of the extruded parison positioned in the open mold;
- figure 2 is a sectional view of the closed mold and of the parison, partially formed
only by means of blow molding: and
- figure 3 shows the closed mold and the parison, definitively formed by the action
of the movable piston.
[0022] With reference to the figures, the device for practising the process of the present
invention comprises a mold, which consists of two parts: a first half-mold (2), concave
and rigid, and a second half-mold (3) containing a movable piston (4) supporting a
template (5) suited to conform to the cavity (6) of the first half-mold (2). The extruded
parson (1), the walls of which are indicated with (A) and (B), is positioned between
the two half-molds.
[0023] While the article is being formed, the half-molds (2) and (3) are closed, and air
is blown into the parison (1) by means of nozzles, which are not shown in the figure.
The parison under goes a deformation and conforms to the surface of cavity (6) and
to the surface of template (5). Simultaneously, or almost simultaneously with blowing,
piston (4) enters the cavity (6), thereby imparting the desired final shape to the
article. During the last-mentioned operation, both surfaces (A) and (B) are subjected
to stretching. On surface (B), stretching is directly exerted by piston (4), while
on surface (A) it is essentially exerted by the pressure of the air, which has remained
entrapped in hollow space (7).
[0024] When carrying the present invention into effect, various changes, modifications and
variations can be brought to the various parts which form the device for preparing
double-wall blown articles having a high stretching ratio, which is illustrated, as
an example, in the figure of the attached drawing.
1. A process for preparing double-wall blow-molded articles having a high stretching
ratio, which comprises:
a) extruding a tubular body with an open end made of thermoplastic polymer through
an essentially annular extruder;
b) positioning the tubular body into an open mold for blow molding, composed of a
first concave half-mold, which partially reproduces the shape of the article to be
produced, and of a second half-mold, which comprises a movable piston supporting a
template capable of conforming to the concavities of the first half-mold; and
c) closing the mold, wherein the front surface of the template is positioned in the
plane of the outer edge of the second-half mold, blow-molding the tubular body, while
it is still in a softened state, to expand the tubular body on the side of the first
concave half-mold and to press the tubular body against the front surface of the template
on the side of the second half-mold, and simultaneously or almost simultaneously introducing
the movable piston of the second half-mold into the concavities of the first half-mold.
2. The process according to claim 1, wherein the extruded tubular body is made of polystyrene,
impact-resistant polystyrene, styrene mixes, polypropylene and impact-resistant polypropylene.
3. The process according to claim 1 or 2, wherein the extruded tubular body has a thickness,
which is a function of the shape and the size of the article to be produced, but generally
ranges from 0.5 to 20 mm.
4. The process according to any of the preceding claims, wherein vacuum is generated
in the concavity of the first half-mold during the molding step.
1. Verfahren zum Herstellen von doppelwandigen blasgeformten Gegenständen mit einem hohen
Streckverhältnis, umfassend:
a) Extrudieren eines rohrförmigen Körpers aus einem thermoplastischen Polymer mit
einem offenen Ende durch einen im wesentlichen ringförmigen Extruder,
b) Anordnen des rohrförmigen Körpers in einer offenen Form für die Blasverformung,
bestehend aus einer ersten konkaven Halbform, die die Form des zu erzeugenden Gegenstands
teilweise reproduziert, und aus einer zweiten Halbform, die einen beweglichen Kolben
umfaßt, welcher ein Templat trägt, das zur Anpassung an die Konkavitäten der ersten
Halbform in der Lage ist, und
c) Schließen der Form, wobei die vordere Fläche des Templats in der Ebene des äußeren
Randes der zweiten Halbform positioniert ist, Blasformen des rohrförmigen Körpers,
während er sich noch in erweichtem Zustand befindet, um den rohrförmigen Körper auf
der Seite der ersten konkaven Halbform zu expandieren und den rohrförmigen Körper
gegen die vordere Fläche des Templats auf der Seite der zweiten Halbform anzudrücken,
und gleichzeitiges oder fast gleichzeitiges Einführen des beweglichen Kolbens der
zweiten Halbform in die Konkavitäten der ersten Halbform.
2. Verfahren gemäß Anspruch 1, worin der extrudierte, röhrenförmige Körper aus Polystyrol,
schlagfestem Polystyrol, Styrolmischungen, Polypropylen und schlagfestem Polypropylen
besteht.
3. Verfahren gemäß Anspruch 1 oder 2, worin der extrudierte, röhrenförmige Körper eine
Dicke aufweist, die in Abhängigkeit steht zur Form und zur Größe des zu erzeugenden
Gegenstands, jedoch im allgemeinen im Bereich von 0,5 bis 20 mm liegt.
4. Verfahren gemäß einem der vorhergehenden Ansprüche, worin in der Konkavität der ersten
Halbform während der Formungsstufe ein Vakuum erzeugt wird.
1. Un procédé pour fabriquer des articles à double paroi moulés par soufflage et présentant
un rapport d'étirage élevé et qui comprend :
a) l'extrusion d'un corps tubulaire ayant une extrémité ouverte réalisé en polymère
thermoplastique, à l'aide d'une extrudeuse sensiblement annulaire ;
b) le placement du corps tubulaire dans un moule ouvert pour réaliser un moulage par
soufflage, constitué d'un premier demi-moule concave, qui reproduit partiellement
la forme de l'article à fabriquer, et d'un second demi-moule comprenant un piston
mobile supportant un gabarit capable de se conformer aux concavités du premier demi-moule;
et
c) la fermeture du moule, dans lequel la surface antérieure du gabarit est positionnée
dans le plan du bord externe du second demi-moule, et la réalisation du moulage, au
moyen d'un moulage par soufflage, du corps tubulaire alors qu'il est encore à l'état
ramolli, pour expanser le corps tubulaire du côté du premier demi-moule concave et
pour presser le corps tubulaire contre la surface antérieure du gabarit du côté du
second demi-moule, et simultanément, ou presque simultanément, l'introduction du piston
mobile du second demi-moule dans les concavités du premier demi-moule.
2. Le procédé selon la revendication 1, dans lequel le corps tubulaire extrudé est réalisé
en polystyrène, en polystyrène résistant aux chocs, en mélanges de styrène, en polypropylène
et en polypropylène résistant aux chocs.
3. Le procédé selon la revendication 1 ou 2, dans lequel le corps tubulaire extrudé présente
une épaisseur qui est fonction de la forme et de la taille de l'article à fabriquer,
mais qui est généralement comprise entre 0,5 et 20 mm.
4. Le procédé selon l'une quelconque des revendications précédentes, dans lequel la concavité
du premier demi-moule est mise en dépression au cours de l'étape de moulage.